A character that could divide an address
An email address has to tell a system two things: which user should receive a message and which computer, or host, should receive it first. Tomlinson used @ as a boundary between those parts: the name before it identified the user, and the name after it identified the host. The symbol’s familiar reading—“at”—made the arrangement easy to understand: a user at a particular host.
Why this mark fit the job
The choice was practical. The mark was already available on the keyboard, so people would not need a new key or a special way to type it. Just as important, it was not normally part of the user or host names in the system he was working with. That made it a useful separator: the software could recognize the boundary without confusing it with either name.
The format also gave the addressing system a simple pattern to follow. When a message was sent, the address could be split at the @: one part indicated the recipient on a host, and the other indicated where to direct the message. The symbol did not itself deliver anything; it made the address legible to both the sending system and the person entering it.
A small choice with a long afterlife
Because the same compact pattern worked across networked computers, it was easy to carry forward as email spread. The @ mark became more than a convenient keyboard character: it turned a potentially confusing address into a readable relationship between a person’s account and the machine that handled it. Each time someone types an email address, that old division still does its quiet work.
